This invention relates to laser beam projection systems utilizing dual light valves to compensate for medium-aberrated waves and to correct for surface defects in a first light valve and, more particularly, to such systems utilizing various arrangements for selectively decoupling interaction between dual light valves.
Since the invention of the laser in the 1960s, a great number of applications have been developed for exploiting the spectral purity and spatial coherence of the laser beam. Communications, data transfer, and the projection and processing of images have come to depend upon the unique properties of the coherent laser wavefront. These properties must be preserved if the powerful and beneficial qualities of laser radiation are to be fully utilized. Except in free space, a laser beam travels through a material medium and the wavefront quality of the laser beam is reduced: high spatial quality waves become aberrated; plane waves emerge with randomly perturbed wavefronts. The diffraction associated with such aberrated waves significantly reduces the ability to focus the beam to a high-quality beamspot or to efficiently transmit a communications signal or image to a remote receiver.
Another problem occurs when such laser wavefronts are transmitting large amounts of energy. Some portion of that energy is absorbed when it passes through a given optics system of lenses, mirrors, and other optical devices, or when it travels through the atmosphere. Typically, when materials absorb energy and heat up, their index of refraction changes. This change in index varies across a given beam profile. The intensity of the beam and the amount of heat absorbed vary as a function of location within the beam. Differences in index cause refraction of a laser beam. The consequent spreading of the high-energy laser beam due to a laser-induced index differential is termed "thermal blooming". Due to thermal blooming, the beam which arrives at its target has spread too far, and even if it is focused, the phase of the beam across its wavefront is so randomized that on the whole it becomes degraded. Thus the laser beam delivers only a small fraction of the energy being transmitted to the receiving site.
In order to counteract these deleterious effects of atmospheric turbulence, thermal blooming, and irregularities within the optical train, adaptive optical systems have been explored and developed. These systems combine wavefront sensing and wavefront correction within a closed feedback loop in order to correct a particular laser beam's wavefront errors. A typical laser beam direction system might work as follows. A laser beam is directed via an atmospheric path to a target or receiving site. Because of turbulence and thermal blooming, only a portion of the radiation reaches the target. In some systems, a laser reference is transmitted back through the atmosphere in order to be used as a probe wave which samples the atmospheric aberrations the light has encountered. In essence, the return signal contains in its wavefront phase the essential information about the composite beam path aberration. If the phase aberrations are then sensed and the laser beam is pre-aberrated to correspond to this phase pattern, during its propagation through the atmosphere, the laser will retrace the path of the target radiation and arrive at the target unaberrated; the full amount of beam energy will then have been transferred.
A variety of apparatus and methods have been developed or proposed for this type of beam correction. These include deformable mirror systems and phase conjugation approaches using nonlinear optical media.
Deformable mirror systems suffer from a great number of inherent problems. The use of discrete, bulky electrical actuators limits the spatial frequency response for the mirror; a deformable mirror simply cannot correct errors finer than the spacing of the push-pull actuator elements. In addition, such actuators typically require several thousand volts for operation and are subject to arc-overs and permanent breakdowns. Their impedance combined with the mass of the mirror surface limits the temporal frequency response of the adaptive system. Each detector/actuator feedback loop requires discrete electronic processing systems and considerable amplification and computation to function properly. Since the thin front surface of the mirror continuously experiences flexures, it suffers from eventual drift and creep problems with consequent loss in performance.
Nonlinear optical media, using degenerate four-wave mixing, stimulated Brillouin or Raman scattering, can provide a time-reversed wave as an output in some applications. While this phase-conjugation method greatly improves the spatial resolution of the system since the "actuator spacing" is now molecular, it also presents certain problems. The input sensitivity is very low, so that a fairly large return signal is necessary in order to set up the proper index grating structure within the phase-conjugation cell. Such power requirements rule out this method for lower-power optical communication and data transfer systems. In addition, with four-wave mixing, coherent pump beams require a considerable amount of energy and must be precisely aligned in angle for the device to function efficiently. The wasted costs of duplicate high-energy lasers for pumping the medium and the resulting low efficiency conversion and transmission of the energy to the target make phase conjugation an interesting but often impracticable means for adaptive optics applications.
An alternative approach to the phase-conjugation method mentioned above involves the use of a liquid crystal light valve incorporating a self-adaptive control system to combine both wavefront sensing and wavefront correction in a single package. Boswell et al in U.S. Pat. No. 4,019,807 disclose one particular version of a standard liquid crystal light valve (LCLV). If the usual liquid crystal element in that disclosed device is replaced with a parallel-aligned tunable birefringent liquid crystal substrate, the modified LCLV device can be used as a deformable mirror. It thus finds particular utility in the wavefront sensing and correction system. A number of benefits flow from the use of such an LCLV in this type of system. The LCLV itself requires no signal processing, electric amplification, or high-voltage sources. It possesses a spatial resolution of approximately 30 microns per pixel, far better than the typical deformable mirror which may have 16 to 60 discrete control areas (or cells) spread over an area of 100 square centimeters.
In such a system, aberrated light from the target, degraded by all the distortions of its travel path, passes through the substituted liquid crystal element, reflects off a dielectric mirror internal to the LCLV, and passes through the liquid crystal element once more. A small fraction of this beam is transferred by various optical means to the back side or photoconductor side of the LCLV where it combines with a locally generated unaberrated, planar, coherent wavefront. This local reference can be realized by spatially filtering part of the feedback beam and, after properly phase shifting it with respect to the initial beam, recombining the beams in the manner indicated in FIG. 1 of the accompanying drawing. The two combined wavefronts create an interference pattern which through careful alignment is in exact registration with the incident target wavefront hitting the front side of the LCLV. A photoconductive layer lies sandwiched behind the liquid crystal element between the dielectric mirror and one clear conductive substrate. Once a voltage is placed across these conductors, any change in resistance in the photoconductor due to absorbed radiant energy engenders a commensurate change in voltage across the liquid crystal element only at that point. The interference pattern impinging on the photoconductive layer provides such a source of radiant energy and the pattern is a representative spatial mapping of the wavefront errors (modulo 2.pi.) of the incoming target radiation. Hence, voltages across the liquid crystal element will change at precisely those points where the phasefront of the target radiation is aberrated. The refractive index and hence optical path length of the liquid crystals will change only at those points corresponding to the applied field variations. Thus, these microscopic phase-shifting elements will retard or advance the local phase of the incoming wavefront until a uniform interference pattern is obtained upon the photoconductive side of the liquid crystal light valve device, at which point the servosystem is in equilibrium.
When the interference pattern is uniform, the incoming wavefront can be completely corrected by the liquid crystal "deformable mirror" if a planar wavefront is reflected off the LCLV. Hence, a pre-aberrated high energy laser beam can be realized so that it can arrive at a given target point substantially unaberrated. Since the LCLV typically requires very little power to produce the desired phase modulation effects, this device can also be used to great advantage in very precise image and data processing systems. Moreover, the rather uniform wavelength dependence of the phase shifts created in the liquid crystal mirror allows multiple wavelength use of the device, just as in conventional deformable mirror technology.
Modified silicon liquid crystal valves (LCLVs) have been selected for use in the system described above for adaptively correcting for atmospheric aberration because of the relatively quick response of the thin silicon device. However, the first generation modified silicon LCLV presents one disadvantage in that it exhibits an inherently high optical non-uniformity. An improved arrangement to compensate for this disadvantage incorporates an additional wavefront correcting LCLV and associated feedback control system to provide correction for the limitations of adaptive optics wavefront correction systems utilizing the thin silicon LCLV.
One of the principal drawbacks of the adaptive optics system using a single or "main" silicon LCLV is that the fast-response, silicon LCLV has a poor surface quality, leading to a reduced output uniformity. The dynamic (phase) range of the device is limited because a relatively thin liquid crystal layer must be used to obtain the fast response. Although the spatial phase-non-uniformity of the silicon LCLV can be corrected by the LCLV valve itself in closed loop operation, this is not a desirable solution because it will use up a significant portion of the already limited dynamic range of the device.
The deficiency of the single LCLV adaptive optics system is compensated for, in the improved arrangement, by the use of a second "corrector" LCLV valve, also operating in a closed-loop configuration, as an adaptive corrector for the main LCLV. The second LCLV has a large dynamic range compared to the main LCLV. Thus it can correct for the main LCLV surface non-uniformities as well as for its own distortions. The LCLV used in the correcting system typically has surface non-uniformities which are much less severe than in the thin silicon LCLV of the main system. This improved arrangement is the subject of application Ser. No. 947,575, now U.S. Pat. No. 5,048,935 mentioned hereinabove. The disclosure of that application is incorporated by reference herein and, further, is here reproduced in part.
In brief, there are two phases in the operation of the aforementioned system. In the first "set mode" phase, the main LCLV feedback loop is disengaged by using an appropriate shutter, and an auxiliary laser is used to drive the wavefront to zero through the auxiliary system coupled to the second LCLV. At the end of the "set mode" period, the wavefront error is driven to zero. At this point, the main operational phase is initiated by activating the main laser, enabling the main feedback loop and disrupting feedback to the compensator LCLV, using appropriate shutters in the second correction loop. Since the corrector LCLV is slow, due to the thick liquid crystal cell which is employed, it will continue to "hold" the modulation information for a certain decay time. Therefore, for the duration of the decay time (which can be up to a number of seconds, whereas the response time of the main laser system is on the order of milliseconds) the main LCLV valve will only have to correct for the atmospheric aberrations, since the corrections for its own aberrations are incorporated in the main wave front. This sequence of " set" and "operation" modes can be repeated with the maximum period being set by the response (decay) time of the corrector LCLV.
This arrangement, by eliminating the necessity of self correction from the main LCLV, allows it to perform correction of the atmospheric aberration faster and with optimal utilization of its limited dynamic range. Dual light valve systems as heretofore disclosed can also be used as general adaptive correctors for high or low spatial frequencies, and for slowly varying errors in existing adaptive optics systems, such as pin cushion errors, optical element variations, thermal distortion of mirrors, and the like.
In the preferred embodiment disclosed in application Ser. No. 947,575, now U.S. Pat. No. 5,048,935 the main LCLV was constructed of silicon, whereas the correcting LCLV was of cadmium sulfide. At the present state of the art, only the silicon type LCLV has the potential for adequate bandwidth for real time atmospheric compensation. The correction LCLV provided a much flatter mirror surface than a silicon device but its speed of response was much too slow, even operating with feedback, for atmospheric compensation.
It has been found that dual light valve systems of the type previously described are faced with a problem of isolation between the respective portions of the system. This may be explained by considering an atmospheric reference beam with an error .phi.atm which impinges on the dual light valve system after the correction LCLV has achieved a compensation for the main LCLV. This will cause the main LCLV to compensate for .phi.atm by reorienting its liquid crystal molecules. The probe laser beam then sees the index change introduced by the molecular reorientation as well as the optical path difference error introduced by the non-flat mirror of the main LCLV. These errors are transferred to the wavefront error sensor of the correction LCLV and the reorientation of its molecules tends to cancel the combined static error. Unfortunately, the dynamic correction capability is cancelled as well; i.e., the atmospheric compensation potential of the combined dual light valve system is also cancelled. What is needed is some isolation technique whereby the correction LCLV does not see the atmospheric compensation which is introduced by the main LCLV but does see the mirror errors which are introduced by the main LCLV.